Solid golf balls, which are excellent in durability and flight distance, have been known as a golf ball. The solid golf balls include a two-piece golf ball having a core and a cover covering the core, and a multi-layered golf ball having at least one intermediate layer between a core and a cover. As a rubber material used for the core of the solid golf balls, there has been widely employed a product prepared by vulcanization-molding a rubber composition that includes: a metal salt of α,β-unsaturated carboxylic acid (co-crosslinking agent) such as zinc acrylate; an organic peroxide, and the like, in addition to a high-cis polybutadiene (BR) as a base rubber, from the viewpoint of stressing the property of resilience.
It is known that the resilience performance can be improved by adding to such a rubber composition an organic sulfur compound (disulfide type (SS), thiol type (SH)) in a very small amount. Some golf balls having an organic sulfur compound-containing core have been already put into practical use. However, details about the mechanism for enabling the performance improvement have not been proven yet.
Patent Document 1 discloses, as the thiol or disulfide type organic sulfur compound, a compound that contains a benzene ring having a substituent, in which the substituent is numerically specified by the substituent constant indicating the electronegativity. It is also shown that compounds with larger substituent constants generally tend to give a higher resilience to golf balls. In Patent Document 1, however, only symmetric compounds (compounds including two identical aromatic hydrocarbon groups each bonded to the S atom of the disulfide) such as bis(2,3,5,6-tetraacetylphenyl)disulfide and bis(pentabromophenyl)disulfide are substantially disclosed as the disulfide compound. Further, a further improvement in resilience performance is also desired.
The resilience performance of the golf balls can be improved by increasing the amount of a sulfur compound to be added within a certain range. However, the use of the sulfur compound above a certain amount tends to adversely cause deterioration of the resilience performance, and also cause decrease in hardness of the golf balls to make it difficult to adjust the hardness, which influences the performance of the golf balls. The hardness can be adjusted (increased), usually by increasing the amount of an initiator or a co-crosslinking agent. However, the hardness adjustment by increasing the initiator amount has limitations, and that by increasing the co-crosslinking agent amount also has limitations because deterioration of the resilience performance tends to be caused.
Further, attempts to improve the resilience performance using a mixture of a plurality of sulfur compounds have been made, but in fact the physical properties derived from the compounds are often poorly exhibited. For example, in use of a mixture of chlorobenzenethiol (with one substituent) and pentachlorobenzenethiol (with five substituents), the resilience performance is slightly lower than the average of the two compounds, and as a result, the effects of the mixing fail to be exhibited.
As mentioned above, it is difficult to provide a golf ball having a further improved resilience performance (flight performance) while attaining a hardness needed for a golf ball even when the conventional organic sulfur compounds are used for the core. Accordingly, it has been desired to provide a solid golf ball that exhibits a higher resilience performance and gives a good shot feeling.
Patent Document 1: JP 3554526 B2